1. Field of the Invention
The present invention relates in general to a working unit of construction equipment such as excavators and loaders. More particularly, the invention relates to a structural improvement in an attachment self leveling linkage suitable to automatically maintain the desired angle of the attachment, such as a bucket or fork detachably mounted to the tip of the working unit, relative to the reference or ground surface, thereby maintaining the horizontal position of the attachment without any motions for controlling the bucket cylinder while lifting the attachment from the initial ground position to its uppermost position by the arm-up motion during the operations of construction equipment.
2. Description of the Prior Art
In typical construction equipment such as loaders, various types of attachments are detachably mounted to the tip of the working unit, thereby effectively performing the desired operations such as digging, loading and lifting operations. The above attachments are selectively pivoted to the tip of the arm which is included in the working unit. The above arm is pivoted to the forehead of the construction equipment's frame. A hydraulic cylinder (hereinbelow, referred to as "arm cylinder") is pivoted to the frame of the construction equipment at its cylinder part and pivoted to the middle portion of the arm at its actuating rod part. Due to the telescopic motion of the actuating rod of the above arm cylinder, the arm can be turned up or down about the pivot joint between the arm and the frame's forehead. In addition, another hydraulic cylinder (hereinbelow, referred to as "bucket cylinder") is pivoted to the arm at its cylinder part and pivoted to the attachment or bucket at its actuating rod part. Due to the telescopic motion of the actuating rod of the above bucket cylinder, the attachment can be turned in and out about the pivot joint between the arm and the attachment, thereby performing the desired operations.
However, the angle of the attachment relative to the reference or ground surface cannot be maintained at a fixed angle while turning up or down the arm by simply operating the arm cylinder without operating the attachment by handling, for example, the bucket control lever. That is, the angle of the attachment relative to the ground surface in the above case gradually varies in accordance with the positional displacement of the arm, thereby failing to maintain the horizontal position of the attachment while turning the arm up or down. Therefore, the operator during the operations performed by a fork used as the attachment must handle the bucket control lever continuously so as to adjust the angle of the fork and to maintain the horizontal position of the fork while turning the arm up or down. This is inconvenient to the operator and reduces the operational efficiency of the construction equipment. That is, the operator must handle the bucket control lever to continuously adjust the angle of the attachment relative to the ground surface simultaneously while handling the arm control lever so as to turn the arm up or down. Such a lever handling motion not only requests the operator to be highly skilled, it also remarkably reduces the operational efficiency of the construction equipment.
In an effort to rectify the above problems, attachment self leveling linkages have been actively studied and developed in the field of the art. The above attachment self leveling linkages are used for automatically maintaining the desired angle of the attachment relative to the ground surface without needing to handle the bucket control lever while lifting up the attachment from the initial ground position, for example, the initial digging or loading position, to the uppermost position during the operations of construction equipment.
Construction equipment with the above attachment self leveling linkage has built up large markets particularly in the Americas. The origin of attachment self leveling linkages for construction equipment goes back to the linkage disclosed in U.S. Pat. No. 2,817,448. However, the attachment self leveling linkage disclosed in the above U.S. patent has a complicated construction as it includes a plurality of connection members with the pivot joints in the same manner as most of the early attachment self leveling mechanisms which were proposed in the 1950's but have not been used practically. Therefore, the attachment self leveling linkage of the above U.S. patent has problems that the linkage is very difficult to produce, increases the cost and weight of the working unit and has a poor operational efficiency.
In addition, other types of attachment self leveling mechanisms using hydraulic circuits instead of the above linkages for maintaining the horizontal position of the attachment have been proposed and used practically. The above self leveling mechanisms using hydraulic circuits perform the same functions as those of the above-mentioned attachment self leveling linkages. However, the operational reliability of the above self leveling mechanisms using the hydraulic circuits are lower than that of the attachment self leveling linkages. In this regard, the self leveling linkages have been used more preferably to the self leveling mechanisms with the hydraulic circuits. In the field of the art, it thus needs to simplify the construction of the attachment self leveling linkages while achieving the desired operational effect of the linkages. When the construction of the attachment self leveling linkages is simplified as described above, the linkages can be easily produced at the lower cost. The self leveling linkages with the simplified constructions also reduce the weight of the working units of the construction equipment, thereby improving the operational efficiency of the equipment.
Representative examples of the attachment self leveling linkages with the simplified construction suitable to achieve the above object are disclosed in U.S. Pat. Nos. 4,486,141 and 5,201,235.
In order to improve the construction of the linkage of U.S. Pat. No. 2,817,448, the attachment self leveling linkage disclosed in U.S. Pat. No. 4,486,141 includes a shorter arm cylinder achieving the same operational effect. However, the self leveling linkage of U.S. Pat. No. 4,486,141 has a problem in that a plurality of link members are concentrated to around the equalizer bar thereby applying overload onto the equalizer bar during the operations of the construction equipment. In addition, the shorter arm cylinder must be pivoted to a lower portion of the equipment's frame. Therefore, the arm cylinder's length is not sufficiently reduced because the arm cylinder should turn the arm up to the desired uppermost position during the operation of the equipment.
Meanwhile, the construction of the attachment self leveling linkage of U.S. Pat. No. 5,201,235 is schematically shown in the accompanying drawing, FIG. 1. As shown in FIG. 1, the above linkage somewhat reduces the number of links and pivot joints between the links. However, both the arm cylinder 302 and the equalizer bar 303 pivoted to both ends of a relatively longer equalizer lever 301 need to be lengthened so as to adjust the attachment within the total operating range. In addition, the longer equalizer lever 301 somewhat projects out of the lower side of the arm 304. The lever 301 thus may strike an obstacle, such as a truck deck, placed ahead of the lever 301 during the operations of construction equipment.